Microlithographic projection exposure methods and systems are used for fabricating semiconductor components and other finely structured components. Use is made of masks (reticles) that bear or form a pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component, such as an integrated circuit (IC). A mask is positioned in a projection exposure system between an illumination system and projection objective in the region of the object surface of the projection objective, and illuminated with illumination radiation provided by the illumination system. The radiation varied by the mask and the pattern forms projection radiation propagating through the projection objective, which images the pattern onto the substrate to be exposed. The substrate normally bears a radiation-sensitive layer (photoresist).
It is known that the image of a pattern provided by the mask can be improved, and the process windows can be enlarged, by appropriate choice of the angular distribution of rays at which the pattern of the mask is illuminated by the illumination system. The angular distribution of rays impinging on the mask is usually adapted to the kind of pattern to be projected onto the substrate. For example, relatively large sized features may involve different angular distribution than small sized features. Ideally, the illumination system illuminates each point of an illuminated field on the mask with rays having a well defined angular distribution.
The term “angular distribution” of rays incident on a point on a mask is generally used to describe which illumination directions (or angles) contribute to the illumination and how the total energy of a ray bundle, which converges towards a particular point in the mask plane, is distributed among the various directions along which the rays constituting the ray bundle propagate. The term “ray” as used in this application corresponds to a “radiation bundle” propagating in a given direction and transporting a certain amount of radiation energy.
In certain types of illumination systems the angular distribution of radiation illuminating the mask is determined by the spatial intensity distribution in a pupil plane of the illumination system, which can be regarded as a secondary (or effective) source. In those systems, particular illumination modes are commonly described by reference to the shape of the spatial intensity distribution in the pupil plane. A set of principal illumination modes (corresponding to a set of principal shapes of the spatial intensity distribution in the pupil plane) is commonly used to characterize various illumination modes. One principal illumination mode is the “conventional illumination”, which corresponds to an even illumination of a mask point from all angles from 0° to a certain maximum angle, and which involves a uniform disk-shaped spatial intensity distribution in the pupil plane. Another commonly-used principal intensity distribution is “annular illumination”, in which the intensity distribution in the pupil plane is an annulus, with no intensity in the center of the pupil. Other principal illumination modes are “dipole illumination”, in which there are two intensity poles in the pupil plane, “quadrupole illumination”, in which there are four intensity poles in the pupil plane, or other multipole illumination settings, such as, for example, having 6 or 9 or more poles.
Within this disclosure, a preselected type of spatial intensity distribution at the pupil plane of the illumination system, which corresponds to one of the principal illumination modes, or to a combination of two or more spatial intensity distributions of principal illumination modes, may be referred to as an “illumination setting”.
To create different illumination settings, various methods have been proposed. For example, a zoom-axicon (a combination of a zoom system and an axicon system) can be used to optionally create conventional illumination with a controllable maximum angle, or annular illumination with controllable inner and outer radii of the annulus. To create dipole and quadrupole type illumination settings, it has been proposed to use spatial filters, i.e. opaque plates with apertures located where the poles are desired, or other light shielding elements. Using spatial filters is often undesirable because the resulting loss of radiation can reduce the throughput of the apparatus.
It has therefore been proposed to employ an optical beam deflecting element, such as for example a diffractive optical element (DOE) or refractive optical element, in the process of forming a desired intensity distribution in the pupil plane of the illumination system. In general, an optical beam deflecting element is placed in the beam path of the illumination system between a primary radiation source associated with the illumination system and the pupil plane forming the effective (secondary) source. In operation, the optical beam deflecting element redirects radiation and thereby changes an angular distribution of rays in a radiation beam incident on the beam deflecting element to generate a radiation beam having rays according to a predefined angular distribution. In other words, the optical beam deflecting element changes in a controlled manner the angular spectrum and energy distribution of radiation interacting with the optical beam deflecting element. In this process, radiation energy is redirected rather than blocked, such that radiation loss can be minimized. By positioning an optical beam deflecting element upstream of the pupil surface and placing a condenser lens system in between, it is possible to produce almost any arbitrary spatial intensity distribution in the pupil surface. If desired, an additional zoom-axicon system makes it possible to vary, at least to a limited extent, the spatial intensity distribution produced by the optical beam deflecting arrangement.
European patent applications EP 0 949 541 A and EP 1 109 067 A (corresponding to US 2003/0214643 A1) describe, among other things, diffractive optical elements in which different regions may have different effects, for example, forming quadrupole or conventional illumination modes so that mixed or “soft” illumination modes can be created.
US 2007/0058151 A1 describes similar diffractive optical elements which further include polarization influencing structures ensuring that radiation redirected into specified directions has a defined polarization.
U.S. Pat. No. 7,292,393 B2 describes a variable illuminator including a diffractive optical element in which different annular sections have different structures to create different spatial illumination profiles at a pupil plane of the illuminator. The sections can be placed selectively into an incident laser beam by rotating the diffractive optical element.
US 2007/0024836 A1 describes illumination systems having three diffractive optical elements which, in combination, influence the angular distribution of a light bundle incident on a mask.
A defined distribution of ray angles in an illumination beam directed at a mask is only one of multiple desired properties for a lithographic process. Other desired properties relate to the distribution of radiation energy (or intensity) over the rays of a ray bundle, which may be characterized using one or more energy distribution parameters.
One property sometimes desire for an illumination system is a good pole balance. The term “pole balance” refers to a parameter which specifies how radiation energy is distributed between two sides of a dividing plane which subdivides the pupil plane centrally. In polar illumination settings, the parameter pole balance (PB) may be employed to quantify how radiation energy is distributed between two diametrically opposed poles of a multilpolar illumination setting.
Another property often desired from highly advanced illumination systems is a good ellipticity. In one definition, the term “ellipticity” refers to a parameter which specifies how radiation energy is distributed along regions centered about two mutually orthogonal directions, such as x-direction and y-direction in a plane perpendicular to the optical axis (z-direction) or about axes running centrally between the x and y directions in diagonal direction. The ellipticity is often used to qualify energy distributions in annular or hexagonal or conventional illumination settings.
Another energy distribution parameter sometimes used to quantify the distribution of radiation energy over the rays of an illumination beam is denoted as “gradient”. The gradient may be defined to quantify a two-dimensional energy disparity, i.e. a variation of intensity along one reference axis in the pupil plane. The variation of intensity along that axis may be described by a parameter which changes along the reference axis.
Another energy distribution parameter sometimes used to describe the distribution of radiation energy over the rays of an illumination beam incident on an illuminated area is denoted as “telecentricity”. In one definition the term “telecentricity” is used in the sense of “energetic telecentricity” and refers to the direction (or angle) in space of an energetic centroid of bundle of rays converging towards a given point in an illuminated area. The energetic centroid represents the average of all propagation directions present in the ray bundle, weighted by the respective amounts of energy (or intensity) travelling in the respective propagation directions. The telecentricity parameter therefore relates to the extent different directions in space contribute to the overall intensity incident on a specific point.
It is understood that any spatial distribution of illumination intensity in a plane can be characterized using one or more of the above energy distribution parameters, and that energy distribution parameters according to other definitions may also be used to qualify the spatial distribution of illumination intensity in the pupil plane, or another property related thereto, such as a balance of radiation energy (or intensity) in a ray bundle converging on a point in the mask plane.
Correction of errors in illumination energy distribution is conventionally achieved by using appropriate attenuation techniques, such as pupil filtering. US 2008/0284998 A1 discloses the use of a variable pupil filter having a plurality of radially movable spokes in combination with a decentering of a lens to correct pole balance errors generated by an illumination system. A variable pupil filter suitable for adapting the illumination angle distribution in a projection exposure apparatus is disclosed in U.S. Pat. No. 6,535,274 B2.
Structured illumination employing particular illumination settings adapted to given patterns to be imaged has helped to significantly improve the quality of the lithographic process. However, it has been observed that a certain quality of the imaging process expected from calculations can not always be obtained even though the same illumination setting is used for the same type of mask and pattern in roughly identical or similar projection exposure systems operated with the same nominal operating conditions, such as operating wavelength λ, image-side numerical aperture NA and other process determining parameters. For example, where large quantities of similar components are manufactured in a manufacturing plant, several projection exposure systems are usually run in parallel performing nominally the same lithographic process to increase throughput. Although similar results would be expected in the printed products, variations in product quality have been observed although the same type of projection systems with the same projection parameters have been used in combination with masks bearing the same type of patterns.